Semiconductor laser diodes are efficient sources of laser radiation. Laser diodes, or more properly, semiconductor lasers, are generally constructed according to well-known principles of semiconductor manufacturing technology. A discussion of these principles can be found in Richard R. Shurtz II, Semiconductor Lasers and LEDs in Electronics Engineers' Handbook, 3rd ed. (hereinafter "Shurtz") (Donald G. Fink and Donald Christiansen, eds. 1989) which is hereby incorporated by reference.
A difficulty in the present art is that semiconductor laser diodes emit highly divergent beam light beams. This presents problems in many applications. The divergence of the semiconductor laser diode's beam is caused by its exit aperture which is very narrow along one axis (the "fast" axis, which is defined to be perpendicular to the laser junction), and much wider along the orthogonal axis (the "slow" axis, which is defined to be parallel to the laser junction). These two axes correspond to the Y and X axes, as will be later explained. The cross section of the beam emitted along the fast, or Y, axis is highly divergent due to diffraction effects. In comparison, the wider aperture, defined along the X axis, emits a beam cross section that diverges only slightly.
In many applications it is advantageous to cause the beam divergence of the emitted light beam to become uniform in the X and Y axes. This is known as "circularizing" the output beam. Much research and effort has been directed towards this end. One particularly successful methodology has been to direct that beam through a particularly configured cylindrical microlens which circularizes the beam. Several patents and patent applications are directed towards the furtherance of this technology. For example, a device of an early type is taught in U.S. Pat. No. 4,731,772, as referenced in U.S. Pat. No. 5,050,153
A method for the fabrication of cylindrical microlenses of the type embodied in the present invention is taught in U.S. Pat. No. 5,155,631, which is hereby incorporated by reference. According the '631 reference, a preferred method for fabrication of cylindrical microlenses starts by forming the desired shape as a glass preform. The preform is then heated to the minimum drawing temperature of the glass, and an elongate fiber is drawn from it. The cross-sectional shape of the elongate fiber bears a direct relation to the shape of the preform from which it was drawn. Elongate strands of cylindrical microlens can be forming in this way. During the drawing process, the lens surfaces so formed become optically smooth due to fire polishing.
In order to collimate the beam produced by a laser diode, the invention taught in U.S. Pat. No. 5,081,639 teaches the mounting of a cylindrical lens optically aligned with the laser diode to provide a beam of collimated light from the Y axis of the diode. The laser diode assembly taught therein includes a diffraction-limited cylindrical lens having a numerical aperture greater than 0.5 which is used to collimate a beam from a laser diode. A collimated beam is one which is neither converging nor diverging; i.e., the rays within the beam are traveling substantially parallel to one another.
U.S. Pat. No. 5,181,224 illustrates the use of cylindrical lenses to (inter alia) create a slowly diverging beam light. This lens may be said to be "circularizing" and, when installed on any of a variety of laser diodes is available as the "CIRCULASER7" diode available from Blue Sky Research in San Jose, Calif.
The U.S. patent application Ser. No. 08/837,002, entitled "MULTIPLE ELEMENT LASER DIODE ASSEMBLY INCORPORATING A CYLINDRICAL MICROLENS", now U.S. Pat. No. 5,963,557 describes another diode/microlens system in which the microlens does not correct for astigmatism of the diode beam, but which is instead corrected downstream with another larger lens or other means. In this system, no active alignment is required to position the microlens adjacent to the laser diode facet, so automation of the process is rendered possible. However, other means are then required to correct for the astigmatism of the beam. These other means take the form of additional optical elements inserted into the beam emerging from the microlens.
U.S. Pat. No. 5,050,153 teaches a device related to the device taught in the '772 patent. In this teaching, the device is implemented as a semiconductor laser optical head assembly utilizing a tilted plate for astigmatism correction in place of the cylindrical lens taught in the '772 reference.
U.S. Pat. No. 5,844,723 ('723) teaches a diode/microlens assembly incorporating two microlenses which circularize and correct for astigmatism. Unlike prior technologies, the microlens assembly of the '723 patent uses microlenses to both circularize and correct for astigmatism. Unlike the present invention, the lenses taught in the '723 patent are a crossed pair of cylindrical microlenses attached to a substrate which is mounted a laser diode chip. The two '723 microlenses are mounted in a crossed-T conformation orthogonal to each other with their flat surfaces facing the emitting facet of the diode, this arrangement requires no additional lenses for astigmatism correction. The crossed pair of lenses can collimate or focus the laser diode beam, for example focusing the beam into a single mode fiber. A difficulty with the crossed pair configuration is the relative difficulty in correctly aligning the lens pairs. Alignment in the crossed-T conformation can not presently be achieved using passive alignment.
To overcome the loss of optical efficiencies inherent in each of these designs, U.S. Pat. No. 5,181,224 addresses the problem by using a single specially constructed cylindrical microlens which circularizes and corrects the astigmatism in the output beam of a semiconductor laser diode. However, to obtain these advantages, this lens must be aligned to tolerances less than 2 microns along at least one axis. This precision alignment has heretofore required the active alignment of the lens with the diode. The resultant apparatus, e.g., the previously discussed CIRCULASER, is a low-divergence, low numerical aperture, highly efficient semiconductor laser diode assembly, with properties unmatched by other laser diodes. It should be noted that each of the above referenced patents are hereby incorporated by reference.
While the previously discussed laser diode assemblies are fully effective for their intended use, the method of their manufacture has heretofore resulted in manufacturing inefficiencies. In any optical system, the alignment of the various optical elements is critical to the functioning of the system. This is certainly the case where a cylindrical microlens is incorporated into an optical system with a laser diode to provide a low-cost source of collimated light. As is typical of many optical applications, there are six degrees of freedom inherent in the positioning of the lens with respect to the laser diode, as shown in FIG. 1. Having reference to that figure, a cylindrical microlens, 100 is shown. The lens has three axes, X, Y and Z. The Z axis, 1, corresponds to the optical axis of the optical system. The X, 3, axis is transverse to the Z axis, 1, in the horizontal plane. The Y, 2, axis is also perpendicular to the Z axis but in the vertical direction. Positioning the lens along the X, Y, and Z axes defines the first three degrees of freedom. Furthermore, the lens may be rotated about each of these axes as shown at 10, 20, and 30, and each of these rotations also defines a degree of freedom with regard to alignment of the lens in the optical system. For cylindrical lenses, placement of the lens along the X axis, 3, is often not critical. In summary, the accurate alignment of a cylindrical microlens with respect to a semiconductor laser diode often requires precise alignment of one with the other with respect to five degrees of freedom.
A fairly typical active alignment methodology generally proceeds as follows: First, a section of cylindrical microlens is mounted on a small mounting bracket which is mounted to a fixture having the ability to be adjusted in the required degrees of freedom. The microlens is then optically positioned and affixed to the semiconductor laser diode. In order to perform these alignments, a laser diode, usually the diode to which the lens will ultimately be assembled, is energized and the diode's laser beam directed through the lens onto a screen or optical sensor. The operator manipulates the lens along and about the several axes until the projected beam meets the required specifications for the assembly. In this manner, movement along the several axes, as well as rotation about those axes is manipulated by an operator who assembles each lens and laser diode. The entire operation is very dependent on the skill of the operator, as the optical cement utilized to affix the lens to the diode introduces a variable into the problem. This variable is engendered by the fact that the surface tension of the cement between the several elements on which it is used causes motion between those elements. This motion of course tends to misalign the optical elements. Active alignment methodologies are generally utilized to produce the devices taught in U.S. Pat. Nos. 5,081,639 and 5,181,224.
The term passive alignment, as used herein, defines a process whereby the lens is aligned with respect to another device solely by mechanical means and thereafter secured in position with respect to the diode or other device. Examples of such mechanical means include mechanical jigs, fixtures, alignment blocks, and the like. Passive alignment does not require the projection of a beam of light through the lens, nor indeed manipulation of the lens with respect to beam alignment or performance. Passive alignment relies solely on the mechanical alignment of the lens with respect to the diode or other device to achieve the required optical alignment.
The term "semi-passive alignment", as used herein, defines an alignment methodology whereby the lens is aligned with respect to another device along at least one degree of freedom solely by mechanical means, i.e., passively. Examples of such mechanical means include mechanical jigs, fixtures, alignment blocks, and the like. Passive alignment does not require the projection of a beam of light through the lens, nor indeed manipulation of the lens with respect to beam alignment or performance. Passive alignment relies solely on the mechanical alignment of the lens with respect to the diode or other device to achieve the required optical alignment. Alignment with respect to one or more of the other degrees of freedom, where required, is effected by an active alignment scheme. The passive and active alignment steps in a semi-passive alignment methodology may be performed in any order.
Preferably, an ideal semi-passive alignment scheme performs the passive portion of the alignment along the most critical degree of freedom. This is often the alignment along the Z-axis. After all alignment is completed, the lens is secured in position with respect to the other device.
What is further needed is an apparatus and methodology enabling passive alignment of complex microlens structures with optical devices, such as laser diodes. Of particular value are apparatus' and methods capable of passively aligning laser diodes to microlens structures having at least one microlens. A particular need exists for apparatuses and methods for aligning two or more microlenses with respect to each other and additionally aligning this multilens structure to other devices, said alignment having tolerances of less than 2.mu. (micron) with respect to one or more degrees of freedom, most particularly along the Z axis of the microlens structures or with respect to individual microlenses.
What is still further needed is a methodology for constructing such structures using passive alignment techniques while being relatively insensitive to changes in final microlens size resulting from pulling errors.
The several references made herein to reference works and to issued and pending patents is to show the state of the art at the time the present invention was made. These references are herewith incorporated by reference.